A patent application Ser. No. Sho 52-38075, Pub. No. Sho 53-123403 entitled "Method for improvement in liquid fuel", has been filed in Japan by the present inventor in which proposed a method for increasing the combustibility of gasoline by several percents is proposed, the method comprising keeping gasoline for several days under a magnetic field of a predetermined intensity. However, it is a problem that the proposed method requires too much time to be practicable.